Anodic pickling and nickel plating of tank interior using single electrolyte



6, 1955 w. H. PRINE 2,726,201

ANODIC PICKLING AND NICKEL PLATING OF TANK INTERIOR USING SINGLEELECTROLYTE Filed Aug. 2, 1950 2 Sheets-Sheet l INVENTOR. WALTER HUGHPRINE'.

ATTORNEY Dec. 6, 1955 PRINE 2,726,201

W. H. ANODIC PICKLING AND NICKEL PLATING OF TANK INTERIOR USING SINGLEELECTROLYTE Filed Aug. 2. 1950 2 Sheets-Sheet 2 WALTER HUGH PRINEATTORNEY United States Patent ANQDIC PICKLING AND NICKEL. FLA-TING 9FTANK INTERHSR USING SENGLE ELECTROLYTE Waiter Hugh Prine, WestHempstead, N. Y.,- assiguor to The International Nickel Company, Inc.,New York,

N. Y., a corporation of Delaware Application August 2, 1950, Serial No.177,300 1 Claim. (Cl. 204-26) The present invention relates to a processfor producing metallic linings on the interiors of large metalliccontainers such as are used in chemical process industries and inrailway and truck tank cars and, more particularly, to a process forproducing electroplated metallic linings in large metallic containersfor transporting and storing corrosive chemicals.

It is well known that large metallic containers are employed as railwayand truck tank carsv for transporting various corrosive chemicals, suchas acid and alkali solutions, petroleum products, and corrosive foodproducts. In transporting corrosive products, it' is necessary to employcontainers of special construction which will be resistant to thecorrosive action thereof. In this connection, the transportation of highpurity alkali and acid solutions in tank car containers has alwayspresented a problem in that it has been ditficult to avoid tank carcontainer corrosion. Of even more importance than the actual corrosionproblem,.it has been very dithcult' to avoid the contamination of.alkalis, acids and other high purity products which are transported withdeleterious elements such as iron, copper, zinc,'etc., which enter thealkalis and acids from the containers because of the corrosive action ofthe products. Caustic soda of high purity is employed in the manufactureof textiles, soaps, paper, etc., and it is necessary to use caustic sodaof high purity, especially caustic soda substantially devoid of iron andother deleterious elements, in order to produce good quality products inthe aforementioned field's of manufacture. For example, in theproduction of rayon a desirable Rayon-grade caustic offrom'about T0 toabout 73 per cent concentration contains iron from about 2 to about 5parts per million. It is obvious that these, com.- mercial requirementsin the rayon industry demand caustic substantially devoid of iron, andit is essential. that during shipment these caustic solutions not pickup substantial iron content.

it was formerly the practice to ship caustic soda solutions of lowconcentration in tank cars. However, this method of shipment was notvery economical, in view of the low concentration of caustic soda andhigh volume of liquid for a given weight of caustic involved. As new andimproved methods were developed in the manufacture of high puritycaustic soda 'soluations of high. concentration, e. g., 50% to 73%concentration, the problem of transportation economics was ameliorated.The transportation of caustic soda solutions of 50% to 73%concentration, while it solved the freight cost problem, only served tointensify the corrosion problem of. the caustic containers used totransport and to store these caustic soda solutions and, in addition,also served to intensify the problem of contamination of the causticsolution with iron and other elements. Presently, solutions of. upwardsto 80%concentration of caustic soda have been transported for somecommercial. uses; but this concentration of caustic soda had to behandled at temperatures up to 200 F., and in some instances as high as275 F., for at ordinary room temperatures concentrated caustic soda offrom about 73% to about 80% concentration is solid; These solutions,when handled hot to facilitate the loading and unloading of tank cars,were-suliiciently-corrosive to'metallic containers to cause undesirablecontamination of the caustic by corrosion products, especially iron.Under these conditions, the contamination pickup in these hot solutionswas considerable. I

Attempts were made to solve this problem by using tank cars of specialconstruction. However, these specially-constructed containers required.the use of special fabrication procedures which added to the final costof the tank cars. Alternative methods were proposed to solve the problemby employing special processing techniques and methods for convertingordinary steel tank cars in situ into corrosion resistant containers fortransporting hot caustic soda solutions of high concentrations. Oneproposed method comprised applying adherent non-metallic coatings, e.g., such coatings as rubber latex or polystyrene compositions, to theinterior surface of the metallic tank cars. However, these coatingsusually did not function properly, usually did not have good servicelife, and, because of the relative weakness of the linings, were-subjectto accidental breaking and cracking in tank car service which isunusually severe as compared to the service required of stationarycontainers. These non-metallic linings were also subject to failurecaused by the subsequent undercutting of the coatings by corrosiveaction of caustic soda solutions. Furthermore, in the chemical processindustries, food industries, etc., a demand for plating in situ largecontainers with nickel has existed; and with increasingprocessdevelopments, the demand for plating large vessels in positionwith nickelhas increased. For example, large storage vessels which werefabricated in position for use in storage ofone non-corrosive chemicalcannot be employed for storage of newly developed corrosive. chemicalsfor the chemicals will be contaminated with constituents from the vesselwalls. Nickel is a desirable passive lining for most metalliccontainers, but ithas been ditlicult to plate these large vessels in'position with nickel. The demand is therefore great'for. a process andapparatus for plating in position these large chemical processcontainers.

Endeavors, have also been made in the application'of nickel linings totank cars in situ and to other large vessels. in position to overcomethe foregoing disadvantages inherent in the use of linings enumeratedheretofore. No satisfactory method has been developed for'applying. inone. manipulation the complete nickel lining to the interior of largecontainers in position and especially to tank cars in situ so as to givesatisfactory result'sf Thus, conventional electroplating methods couldnot be em ployed. successfully to produce good adherent corrosion:-resistant metallic linings onthe interior of these large containers inposition. For example, when coating small articles with nickel byelectrodeposition from a nickel electrolyte, the usual procedurecomprised cleaning the articles in an alkaline type of cleaningsolution, i. e., caustic solution, to free them of organic contaminatingsubstances, rinsing with water to remove the alkali, and then removingall oxide and objectionable metallic-oxide films in a separate aqueoussolution by'anodic'pickling or by other suitable pickling means,removing said article from the solution and washing it free fromadhering pickling solution and pickling sludge, transferring saidarticle to a nickel electrolyte, and plating thereon a deposit of.nickel. However, the foregoing plating, procedure was not successfulwhen employed to produce satisfactory nickel deposits on large metalliccontainers such as are employed in tank cars. The transfer time requiredfor rinsing with water and replacing large vol umes of picklingsolution, with. large volumes of plating solution in large containersand especially tank cars was considerable, and thispermittedre-oxidationof the pickled steel. As a result, the adhesion ofthe nickel electroplate obtained by this plating method requiring.removal of pickling solutions, water rinsing, and replacement withplating solution was poor. Furthermore, the electroplate flaked badlyand did not prevent corrosion of the underlying surface. realized thatthere is a need for successfully obtaining a nickel lining within largecontainers in position, and especially tank cars. The art has realizedthat a lining is needed which will not only be resistant to corrosion bychemicals, such as hot concentrated caustic soda solutions, foodproducts, etc., but which would also be strongly adherent and thereforebe capable of withstanding the rough treatment to which tank cars aresubjected. In addition, a lining is needed which will be capable ofgiving good service life.

I have discovered a process whereby a strongly adherent,corrosion-resistant lining of nickel can be applied to the interiorsurface of large steel containers, such as tank cars, by electroplatingin situ, which lining when applied overcomes the objectionable featuresof the prior art discussed hereinbefore.

It is an object of the present invention to provide a large containerlining which is resistant to corrosive chemicals, such as concentratedsolutions of caustic soda, and petroleum products.

Another object of the invention is to provide a corrosion-resistantmetallic tank car lining which is strongly adherent and which possessesgood service life.

The invention also contemplates providing an electrolyitc process forapplying a corrosion-resistant metallic nickel lining to largecontainers, such as are employed in tank cars, having good rupturestrength and being capable of withstanding the usual rough treatment towhich tank cars are subjected.

It is a further object of the invention to provide an electrolyticprocess for applying a corrosion-resistant lining in situ to largecontainers such as used in tank cars, chemical process industries, andthe like.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the drawings, in which:

Figure 1 represents a diagrammatic illustration of an installation suchas is employed in carrying out the present invention;

Fig. 2 is a plan view of the particular anode structure showing indetail the anode structure employed in carrying out the practice of thepresent invention;

Fig. 3 is a sectional view taken on line 3-3 of Fig. 1; and

Fig. 4 is a sectional view taken on line 4-4 of Fig. 1.

Generally speaking, the present invention contemplates a process for theapplication of a corrosion-resistant metallic nickel lining to metalliccontainer interiors in situ, i. e., iron, steel, etc., containers. Thecoating consists of a continuous adherent nickel lining which is appliedelectrolytically by employing a nickel-containing electrolyte which iscontained within the metallic container. Broadly stated, the processcomprises mechanically cleaning the interior surface of a largecontainer, such as used in tank cars, by suitable means, such as sandblasting, shot blasting, etc.; suspending in said container ademountable skeletonized electrode structure adapted to distributecurrent substantially uniformly over the interior surface to be plated;filling the interior of the container with an acid nickel-platingelectrolyte; anodically pickling the interior of the container byemploying the nickelplating electrolyte as the pickling solution and bypassing an electric current from the tank car container through theelectrolyte within the tank car container to the suspended electrodestructure, i. e., make the tank anodic with respect to the electrodestructure; and without removing the electrolyte from the container,reversing the current to pass a plating current from said suspendedelectrode structure through said nickel electrolyte to the cleaninterior surface of the container to electroplate nickel from saidelectrolyte onto the interior surface of the tank car container whileagitating said electrolyte by suitable means. The foregoing process inIn view of the foregoing, the art has 2,726,201 t t t sures theproduction of a strongly adherent uniform nickel lining in situ. Bycarrying out the invention in the aforementioned manner, re-oxidationand passivation of the cleaned steel of the interior surface of themetallic container subsequent to the pickling operation are preventedand a very adherent electroplate of nickel is obtained having goodcorrosion resistance and being capable of giving good service life.

Referring to the drawings, a diagrammatic illustration of aninstallation such as may be employed in carrying out the practice of thepresent invention is shown wherein reference character 8 represents atank car container. Assembled within the tank car container 8 is ademountable skeletonized anode structure having an externalconfiguration substantially that of the internal configuration of tankcar container 8. The anode structure consists of a supporting centralspine anode support 10 having an insulated hook 11 attached thereto andinternally threaded collars 34 and 35 welded thereon. The central spinesupport 10 is rigidly connected to an upper internal longitudinal anodesupport 12 and a mid-section lateral anode support connecting member 27.Longitudinal anode support 12 is rigidly connected to the central spinesupport by being sectioned into two pieces and being threaded into theinternally threaded collar 34. Also, the connecting member 27 issectioned into two pieces and is rigidly connected to central spinesupport 10 by being threaded into internally threaded collar 35. Eachend of the mid-section lateral support connecting member 27 is rigidlyconnected to substantially the midsections of side longitudinal anodesupports 25 and 26. Side longitudinal anode supports 25 and 26 are eachsectioned in two sections to provide ease of insertion and removal. Thesections are joined together by being threaded into internally threadedcollar 36 to provide complete rigid members 25 and 26 within tank carcontainer 8. The ends of the longitudinal anode supports 25 and 26 arerigidly connected at substantially the midsections of end arcuate anodes32 which being assembled substantially define the end circumferentialcontours of the tank car container 8. Preferably, anode supports 10, 12,25 and 26 are insulated. Vertically rigidly connected along the planesurface of end arcuate anodes 32 are anodes 29, said anodes 29 beingspaced and arranged so as substantially to define the end portion areasof tank car container 8. Alternately spaced and rigidly connected to thelongitudinal anode supports 12, 25, and 26 are arcuate anodes 13substantially defining the entire longitudinal internal contour of thetank car container 8. The individual members of the entire anodestructure are rigidly attached one to the other by connecting pins 33 orby any other connecting means, such as bolts, screws, etc. The assembledanode structure as described hereinafter is partially supported andcentrally located within the tank car container 8 by insulated hooks 15connected in position onto the upper inner surface of the tank carcontainer 8 substantially near the ends of the longitudinal anodesupport 12. The large spine hook 11 rigidly connected to the centralspine 10 supports most of the anode structure weight. The largesupporting hook 11 is positioned by any supporting means. An electricalcircuit 30 is connected to the central spine support 10, the tank carcontainer 8, and an electrical generator 18. An electrolyte transferfluid circuit is provided through tank car container 8, conduit piping23, including a shut-off valve 17, a force pump 19, a filter 20, and anauxiliary electrolytic cell 22 to remove iron from the electrolyte,connected in series and in the order named. The electrolytic cell 22 hasan electrical circuit 31 connected to an auxiliary electrical generator21. The tank car container 8 is provided with supporting hydraulic jacks16 at each end of the tank car container. A manhole opening 9 isprovided with a removable extension 14 having a circumferential contourequal to the circumferential contour of manhole opening 9.

In performing a nickel-plating operation, upon. the internalsurfaceoftank car container 8, the container is first mechanicallycleaned as by -shotblasting, sand blasting, etc. The anode structuredescribed hereinbefore is then assembled, suspended,.and-supportedwithintank car container 8 by hooks 11 and 15 and thecontainer. filled to thetop of extension 14 with a nickelplating electrolyte. Current fromgenerator 18 is passed from the tank car container 8 through the.electrolyte, through the anode structure as described hereinbefore, andthrough the electrical circuit 3.0 back to electrical generator 18. Thiselectrical current is provided fora timerequired to pickle the interiorsurface of tank, car container 8. This electrolytic picklingoperationusing the plating electrolyte cleans thesurface as describedhereinafter. While this pickling operation is performed, it is preferredto rock the tank car container 8 by alternately actuating the hydraulicjacks 16. During the pickling and plating operations, valve 17 isopenedand pump 19 is operated to circulate-the, electrolyte. through thefilter 20 and the electrolytic cell 22' This. continuous circulation ofthe elect-rolytethrough' the pump 19,. filter 20, and cell 22continuously provides agitation of the electrolyte. The action of theforce pump 19 transfers the electrolyte in tank car container 8 through;filter 20 wherein sludge, sediment, and any other mechanical particlesare removed. The electrolyte then passes through electrolytic cell 22and auxiliary electrical. generator 21 is operated to produce anelectrolytic current through. cell 22. Iron and other metallicimpurities, which have been picked up in the pickling operation in tankcar container 8, are plated out at the cathodes in cell 22,.and theelectrolyte passes through conduit 23 back into container 8 throughmanhole opening9. After thepickling operation is performed, the. platingoperation is begun. The current is reversed from the. electrical currentdirection employed in the pickling-operation so that the current nowflows to and through the anode structure to the: tank car container 8,through the electrical circuit: 30 back. to electrical generator 18. Byemployingthis' current flow; nickel is plated from the electrolyte.intank car container 8. onto the clean pickled interior surface. of: thetank car container 8 tov form a nickel-plated liningthereone Tank carcontainer 8 is rocked by hydraulic jacks 16 during the plating operationto remove anygas pockets and to agitate the electrolyte. Continuouscirculation-ofv the electrolyte through conduit 23' also-provides?agitation. After the plating operation hasi'beenperforrned, the elec'trolyte is pumped out of the tankzcarx container 8 and. the anodestructure is disassembledtand removedtogbeuse in other platingoperations. 1

It is an essential feature of the present invention that the anodicpickling operation and the nickel electroplating operationbeaccomplished consecutively'in the same electrolytic bath withoutremoving thebath, from the tank car. Inorder to obtain an adherentnickel plating, it is necessary that the steel surfaces on which thenickel plating is to be electrodeposited' be clean, and free "of oxidefilms, etc., which produce pooradhesion between the tank surface. andthe electrodeposit. The interior of theta'nk car is first mechanicallycleaned, e; g., by sandblasting, shot blasting, etc., to remove themech-anicallyadherent oxides, such as mill scale, coarse rust,. etc.Thereafter, the, surface is treated by'anodic'pickli'ng in thenickelplatingv electrolyte which is employed in the process toremovelthe microscopic surface'filmswhich passivate. the surface andinterfere with the production of an adherent nickel plate. In order topreventthe formation of other passivating films, after the anodicpickling, operation required. by. the invention, the electroylte iskeptlin. constant protective contact with the cleaned'container surface.

In carrying the invention: into practice, an acid nickelplating bath.selected. fromthe group. consisting: of the sulfate-chloride baths;andwchloride bathsmay be; em: ployed successfully. For example, goodnickel adhesion is; obtained when using an aqueous: plating bathcomprisingabout 150 to about 500 grams per liter of nickel sulfate, fromabout 20 to about 65 grams per liter of nickel chloride, and from about15 to about 45 grams per liter of boric acid, the pH of said bath beingmaintained from about 10m about 4.5. Another satisfactory plating bathcontains about 300 grams per liter of nickel chloride and about 30 gramsper liter of boric acid and has a pHof from about 1.0 to about 3.5. Apreferred plating bath containsabout 150 to-about 300' grams per literof nickel sulfate, from about 15 to about 30 grams per liter of nickelchloride, about 30 grams per liter of boric acid and has a pH maintainedatabout 2 (electrometric).

When one of the foregoing solutions is used for the dual purpose ofanodically pickling and nickel plating the interior surface of a tankcar, the pH range of the solution is maintained at about 1.0 to about4.5 (electrometric). However, for bestresults, it is preferred-that thepH range of the solution bemaintained as low as is consistent withobtaining good nickel. deposits, e. g., at a pH of about. 2.0.. Thecurrentdensity which is employed during the anodic pickling operationmay range from about. 5 amperes per square foot (a. s. f.) to about 50a. s. f., and preferably at about 10a. s. f. Too high a current densitywill result in an excessive dissolution of iron into the platingsolution. The time interval in the operation during which the steelsurface is de-passivated and electrolytically cleaned depends on thecharacter of the steel surface, bath temperature, current density, etc.In general, when employing the foregoing current densities, the anodicpickling time may range from about 15 seconds to about seconds, andpreferably about 30 seconds. It is desirable that the anodicpicklingtime should not be below about 30 seconds at a current. densityof '10 amperes per square foot in order to secure desirable pickling ofthe container. interior. The upper limit of pickling timev is dependentupon variousfactors, e. g., type of material, solution concentration,the need for minimizing iron pickup in the electrolyte, etc. When theinterior surface ofv the tank. car has been sufficiently pickled, thecurrent is reversed, the tank car is made cathodic, and nickel iselectroplated thereon. When long pickling times are used during theanodic pickling operation, it is preferred not to reverse the currentimmediately butrather to stop the pickling, operation for a sufficientlength of time in order to allow the anodic film which forms next to theinterior surface of the tank car to diffuse into the solution body. Thisholding period between the anodic and cathodic application of current tothe tank surface can beshortened or dispensed with by continuouslyagitating the solution during the anodicv pickling operation. Holdingperiods of up to about 60 seconds may be usedas desired, although longerperiods of time. may be used without any detrimental effects. With. goodsolution agitation, a holding period of at least 3 seconds isadvantageously employed to provide diffusion of substantially all theanodic film into the solution body. This time may be shortened byincreasing the degree of agitation. The electrolytic bath temperaturefor the anodic pickling operation should be maintained within the rangeof about70 F. to about 160 F. The temperature of the electrolyte. duringthe plating operation may be maintained from about 70 F. to about 160 F.with an optimum temperature Within the range of from about 70 F. toabout 90 F. when employing a current density of. about 10 a. s. f. If itis. desirable to perform the plating quickly, a faster plating rate maybe provided by heating the electrolyte to a temperature from about F. toabout F. and employing a current density 'within the range of from about40 amperes. per squarefoot to about 50 amperes per square foot.

Upon completion of the anodic pickling operation, the tank car is madecathodic by reversing the current and nickel is. electroplated onto theinterior surface ofv the tank car without removing the nickel solutionfrom the tank car. The current density employed during the platingoperation is controlled within the range from about a. s. f. to about100 a. s. f., and preferably at about a. s. f. when the lowertemperature range is employed. The plating operation is continued untila thickness of at least about 0.01 inch is obtained which may beincreased to about 0.06 inch, depending upon the length of time thecurrent is applied. In practicing the plating operation, a plating timeof about hours at a current density of 10 a. s. f. will generallyprovide a deposit of nickel of 0.01 inch and if a current density ofabout 50 a. s. f. is used, a plating time of about 4 hours will providea 0.01 inch deposit of nickel.

In order to obtain a uniform nickel plate on the interior surface of atank car, it is necessary to use an anode or anodes having, in general,an external contour configuration substantially that of the interior ofthe tank car so as to maintain a substantially uniform distance betweenthe anode and the cathode surface area and also to effect a uniformdistribution of current and uniform deposit thickness during the platingoperation. Insoluble anodes, such as lead, or soluble nickel anodes maybe used in carrying out the plating operation of the invention. Ifinsoluble anodes are used, frequent additions of nickel ions should bemade to the nickel bath by adding nickel carbonate or by solution ofnickel in an auxiliary regenerative unit. In order to compensate for thenickel ions depleted in the bath during the plating operation, it ispreferred to use soluble nickel anodes supported on wrought nickelsupports which are insulated except where the anodes make contact withthe anode supports. It is not contemplated that the anodes will beremoved during the plating operation, and the solution within the tankcar container should be rocked at intervals in order to displace anyhydrogen gas which may tend to be trapped as pockets under the topsurfaces of the container during the plating operation. Rocking can beaccomplished with the use of any suitable rocking means, such ashydraulic jacks. The solution can also be agitated by circulationpumping by using any suitable pumping means to provide a circulation ofthe solution.

In the application of the process described hereinbefore to SAE 1030steel using an electrolytic bath consisting of 300 grams per liter ofnickel sulfate, 30 grams per liter of nickel chloride and 30 grams perliter of boric acid, said electrolyte having an acidic concentration atwhich the pH of the bath was about 2.0 and being at a temperature ofabout 70 F., a continuous nickel plate was obtained having an adhesionvalue of about 54,000 pounds per square inch. An electrolytic bathconsisting of 150 grams per liter nickel sulfate, 15 grams per liternickel chloride and 30 grams per liter boric acid, said electrolytehaving a pH value of about 2.0 provided a nickel lining having anadhesion value of about 91,000 pounds per square inch. In carrying outthe anodic treatment step of the process, the current density wasmaintained at about 10 amperes per square foot for about 30 seconds. Acathodic plating operation was carried out in each case with the currentbeing maintained at about 10 a. s. f. until the nickel linings, havingthe adhesion values described hereinbefore, were secured.

The present invention is particularly applicable to tank car containersused in shipping corrosive chemicals, such as solutions of caustic soda,acids, petroleum products, chemicals and other food products. By usingthe process of the invention disclosed hereinbefore, it is possible toconvert tank car containers heretofore not usable for transportingcaustic soda and other highly corrosive substances into corrosionresistant containers by electroplating the interior surface of tank carswith a strongly ad herent, corrosion-resistant lining of nickel. Thenickel electroplate obtained by the process described herein- 7 beforeis continuous throughout the interior of the tank car and is uniform incomposition throughout. Furthermore, the present invention isparticularly applicable to plating metallic containers in positionwithout disassembly or refabrication of the metallic container in orderto perform the plating'operation.

It is to be noted that the present invention is not to be confused withprior methods of obtaining by electrolytic means strongly adherentnickel plate, in that in this inventive process the anodic cleaning stepemployed in the process is carried out in the same bath from which thenickel electroplate is obtained without removal of the electrolyte bathfrom the container during the pickling and plating steps.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claim.

I claim:

A process for electrolytically depositing a thick, corrosion-resistantlayer of nickel on the interior surface of a large steel tank in situwhich comprises, in combination, the steps of assembling and suspendingan electrode structure in the cavity defined by the interior surface ofsaid tank, filling said tank with an aqueous acid nickel-platingelectrolyte having a pH of about 1 to about 4.5 and containing about toabout 500 grams per liter of nickel sulfate, about 20 to about 65 gramsper liter of nickel chloride and about 15 to about 45 grams per liter ofboric acid, passing an electric current for at least 15 seconds throughsaid tank, electrolyte and electrode structure in a direction such thatthe interior surface of said tank contacted by said electrolyte becomesthe anode and is anodically pickled over substantially all portionsthereof simultaneously, subsequently passing an electric current throughsaid tank, electrolyte and electrode structure in a reverse directionsuch that the interior surface of said tank contacted by saidelectrolyte becomes the cathode and nickel is electrolyticaly depositedover substantially all portions thereof simultaneously, maintaining thecurrent in the latter direction until a corrosion-resistant layer ofnickel of a thickness of at least about 0.01 inch is built up byelectrolytic deposition on substantially all portions of the interiorsurface of said tank contacted by said electrolyte, and removing saidelectrolyte from said tank, whereby a thick, corrosion-resistant layerof nickel is electrodeposited on the interior surface of said tank whileusing the same acid nickel-plating electrolyte for the dual purpose ofanodically pickling and of electroplating the interior surface of thetank without emptying the electrolyte from the tank between the picklingand plating operations.

References Cited in the file of this'patent UNITED STATES PATENTS OTHERREFERENCES Am Electrochem. Soc., special vol. on Modern Electroplating,pp. 239-244 and 246249, 1942.

